A typical form of an internal-gear pump without a filling member therein comprises a casing with a bearing ring accommodated in a bore in the casing movably transversely with respect to its axis but non-rotatably. An internally toothed annular gear is mounted rotatably in the bearing ring; a pinion which is rotatably mounted in the casing has its teeth meshing with the annular gear, defining a suction chamber and a pressure chamber. In an internal-gear pump of that kind, as is to be found in DE 195 17 296 A1, the bearing ring in which the annular gear rotates is accommodated in the bore in the casing, with a radial play of about 0.2 mm. The bearing ring is movable transversely to its axis, to the extent of that radial play, but the bearing ring is prevented from rotating by means of a stud bolt which is disposed on the suction or intake side of the casing. Provided on the pressure or discharge side of the casing in the wall of the bore in the casing is a shallow recess in which is defined a number of pressure areas which communicate with the pressure or discharge chamber defined by the tooth arrangement, by way of radial openings in the bearing ring and radial openings in the annular gear.
The pressure forces prevailing in the pressure chamber of the tooth arrangement have the effect that the annular gear seeks to move away from the pinion. That gives rise to the tendency that the sealing contact which exists to delimit the pressure chamber from the suction or intake chamber, as between the tips of the teeth of the pinion and the annular gear in an engagement-free region of the annular gear in which the teeth of the pinion have come practically completely out of the gaps between the teeth of the annular gear, decreases or is entirely lost. That tendency however is resisted by the pressure force which is produced by the pressure areas and which causes the bearing ring and together therewith the annular gear to be displaced towards the suction side, within the limits of the above-mentioned available radial play. By virtue of that mobility of the bearing ring with the annular gear, the sealing contact as between the tips of the teeth of the pinion and the annular gear is maintained, proportionally to the pressure obtaining on the pressure side.
The provision of pressure areas in a recess in the casing and communicating same to the pressure chamber of the tooth arrangement of the assembly is relatively complicated and therefore increases the manufacturing costs of the internal-gear machine such as a pump. Furthermore the openings which are provided in the bearing ring on the pressure side and by way of which pressure is applied to the pressure areas form a non-homogeneity in regard to the loading on and the deformation of the bearing ring, and that can have an adverse effect on the rotary movement of the annular gear in the bearing ring.